THE MECHANISM OF SPECTRAL SHIFT AND INHOMOGENEOUS BROADENING OF AN AROMATIC CHROMOPHORE IN A POLYMER GLASS

We have attempted to obtain microscopic-level understanding of the abs orption band of the chromophore s-tetrazine in a glassy polymer matrix (atactic polypropylene) at low temperatures. Our investigation has fo cused on the bathochromic shift of the lowest-energy pi <-- n (B-1(3u ) <-- (1)A(g)) electronic transition of the chromophore in the polymer matrix as well as on the width of the inhomogeneously broadened absor ption band. The absorption spectrum of s-tetrazine in atactic polyprop ylene was measured over a range of temperatures for comparison with mo deling results. Information on the geometry and the electronic structu re of s-tetrazine in the ground and excited states was obtained from a b initio calculations. We have generated several polymer microstructur es with imbedded chromophore molecules and used classical NpT molecula r dynamics simulations to obtain ground-state trajectories of the chro mophore. The classical Franck-Condon principle was invoked to calculat e the average solvent shift. The dominant dispersion contribution to t he solvent shift was calculated both, using an empirical parametrizati on of pair-potentials for the excited state (Kettley et al. Chem. Phys . Lett. 1986, 126, 107-12) and using the semiempirical theory of Shale v et al. (J. Chem. Phys. 1991, 95, 3147-66). The very encouraging resu lts obtained indicate that the concurrent use of ab initio calculation s, semiempirical methods, and classical simulation techniques can prov ide valuable insights into the complex microscopic interactions in low -temperature amorphous materials. It is also anticipated that such com putational investigations may become valuable supplements to line-narr owing and single-molecule spectroscopic investigations of amorphous po lymers at low temperatures.